Understanding of light dispersing elements in a spectrometer

Dr. Deepak

Dispersion of light through prism

It is a well-known fact that ordinary white light comprises of a mix of different wavelengths or colours. We are all familiar with the different colours of the rainbow which are in fact different colours comprising white light. White light is seldom used as such in spectroscopic analysis though of visible light source is a common component of a UV – VIS spectrometer. Untreated white light from such a source provides little useful information on the chemical composition of materials.

The incident beam of white light needs to be resolved into its constituent wavelengths before it can provide relevant chemical information on molecular structure and composition of materials. The basic property of maximum absorbance at specific wavelengths by different chemical entities is the basis of chemical estimation in routine laboratory analysis using the rapid optical spectroscopic analysis techniques.

Prisms

A prism is a triangular block of glass or quartz with smooth polished surfaces which is used to disperse the incident light beam into its constituent wavelengths or colours.

Glass absorbs light in the UV region and is not used for absorbance studies in the wavelength range from about 200- 350 nm wavelength range. On the other hand quartz can be used over both UV and visible regions. However, though prisms offer lower-priced dispersion medium they suffer from nonlinear dispersion at longer wavelengths, i.e. the, at the upper end in the visible region, say, from 600 – 800 nm and the dispersed wavelengths appear to be clubbed close together

Gratings

Gratings are plane surfaces on which grooves are etched and the distance between the etched grooves or lines are of the order of wavelengths of the light to be dispersed. The gratings are costlier than prisms but offer advantage as the dispersed light is free from any non-linear distortions over the wavelengths dispersed.

Monochromator

A monochromator is an arrangement of optical elements used in a spectrometer to isolate the desired wavelength bands for carrying out spectroscopic analysis.

The commonly used monochromatic configuration also known as Czerny- Turner grating monochromator is described briefly. The main components comprise of:

Entrance Slit – the broadband light from the light source is collimated to a narrow beam depending on the width of the entrance slit which is then led to a concave mirror that reflects and spread the beam over the surface of the grating.

Grating – The grating disperses the incident beam into its constituent wavelengths. A fixed grating disperses the incident beam in a definite pattern. On the other hand the grating can be rotated on its central axis to spread the diffraction pattern to cover broad wavelength bands. The reflected dispersed beam is led to the second concave mirror

Exit Slit – the reflected dispersed light reaches the exit slit which is located at the focal plane of the second concave mirror. The exit slit width can be fixed or can be varied to get the increased intensity of light for sensitive determinations. However, the exit slit width needs to be optimized to get the best of higher beam intensity and at same time to avoid the spread over the band of wavelengths of the isolated beam.

In order to isolate the wavelength band of interest and to select the optimum width of exit slit it is always helpful to carry out trial absorbance measurements before taking the final observations under optimum conditions